Optical gratings are important elements for selectively controlling specific wavelengths of light within optical systems. Such gratings include Bragg gratings, long period gratings and diffraction gratings and typically comprise a body of material and a plurality of substantially equally spaced optical grating elements, such as index perturbations, slits or grooves.
A typical Bragg grating comprises a length of optical waveguide, such as an optic fiber, including a plurality of index perturbations substantially equally spaced along the waveguide length. The perturbations selectively reflect light of wavelength λ equal to twice the spacing Λ between successive perturbations as multiplied by the effective refractive index, i.e., λ=2neffΛ, where λ is the vacuum wavelength and neff is the effective refractive index of the propagating mode. The remaining wavelengths pass essentially unimpeded. Such Bragg gratings have found use in a variety of applications including, among others, filtering, adding and dropping signal channels, stabilization of lasers, reflection of fiber amplifier pump energy, and compensation for waveguide dispersion.
A long period grating typically comprises a length of optical waveguide wherein a plurality of refractive index perturbations are spaced along the waveguide by a periodic distance Λ′ which is large compared to the wavelength λ of the transmitted light. Diffraction gratings typically comprise reflective surfaces containing a large number of parallel etched lines of substantially equal spacing. Light reflected from the grating at a given angle has different spectral content dependent on the spacing. The spacing in conventional diffraction gratings, and hence the spectral content, is generally fixed.
A common difficulty with all of these grating devices is temperature sensitivity. In Bragg gratings, for example, both neff and Λ are temperature dependent, with the net temperature dependence for a grating in silica-based fiber being approximately +0.0115 nm/° C. for a wavelength λ=1550 nm. The temperature-induced shift in the reflection wavelength is primarily due to the change in neff with temperature. While such a temperature-induced wavelength shift can be avoided by operating the grating device in a constant temperature environment, this approach requires the use of expensive and relatively bulky equipment to maintain the constant temperature.
U.S. Pat. No. 6,148,128, issued to S. Jin et al. on Nov. 14, 2000 discloses a passive temperature-compensated tunable optical fiber grating, where the grating is fixed within a stationary frame and a fiber-flexing movable body is disposed above the fiber to “flex” the fiber and induce a tensile strain so as to alter the resonant wavelength of the device. In particular, the movable body is magnetically (or mechanically) actuated to press against the fiber grating to provide a predetermined strain.
U.S. Pat. No. 6,295,399 issued to J. W. Engelberth on Sep. 25, 2001 discloses a different type of temperature compensating device for a fiber grating, using fiber and second expansion members having different coefficients of thermal expansion. The expansion members are elongated in a direction parallel to the fiber grating, with levers secured to both ends of the expansion members. Each lever has a first end flexibly secured to a respective end of the first expansion members and a middle portion flexibly secured to a respective end of the second expansion member. The other end of each lever is secured to a respective end of the fiber grating through a respective quartz block. The dimensions of the expansion members and the quartz blocks, as well as the materials of the expansion members, are selected to achieve a non-linear temperature response of the fiber grating.
While these and other devices have been useful in providing temperature compensation for fiber gratings, the devices are in general rather large in size and oftentimes cumbersome to operate. What is needed, therefore, is an arrangement for providing temperature compensation to a fiber grating that is relatively small and easy to package.